1. Technical Field
The present invention generally relates to a marine diesel cylinder lubricating oil composition, in particular, for lubricating a marine two-stroke crosshead diesel cylinder engine.
2. Description of the Related Art
In the not so distant past, rapidly escalating energy costs, particularly those incurred in distilling crude oil and liquid petroleum, became burdensome to the users of transportation fuels, such as owners and operators of seagoing ships. In response, those users have steered their operations away from steam turbine propulsion units in favor of large marine diesel engines that are more fuel efficient. Diesel engines may generally be classified as low-speed, medium-speed, or high-speed engines, with the low-speed variety being used for the largest, deep shaft marine vessels and certain other industrial applications.
Low-speed diesel engines are unique in size and method of operation. The engines themselves are massive, the larger units may approach 200 tons in weight and an upward of 10 feet in length and 45 feet in height. The output of these engines can reach as high as 100,000 brake horsepower with engine revolutions of 60 to about 200 revolutions per minute. They are typically of crosshead design and operate on the two-stroke cycle. These engines typically operate on residual fuels, but some may also operate on distillate fuels that contain little or no residue.
Medium-speed engines, on the other hand, typically operate in the range of about 250 to about 1100 rpm and may operate on either the four-stroke or the two-stroke cycle. These engines can be of trunk piston design or occasionally of crosshead design. They typically operate on residual fuels, just like the low-speed diesel engines, but some may also operate on distillate fuels that contain little or no residue. In addition, these engines can also be used for propulsion, ancillary applications or both on deep-sea vessels.
Low- and medium-speed diesel engines are also extensively used in power plant operations. A low- or medium-speed diesel engine that operates on the two-stroke cycle is typically a direct-coupled and direct-reversing engine of crosshead construction, with a diaphragm and one or more stuffing boxes separating the power cylinders from the crankcase to prevent combustion products from entering the crankcase and mixing with the crankcase oil. The notable complete separation of the crankcase from the combustion zone has led persons skilled in the art to lubricate the combustion chamber and the crankcase with different lubricating oils.
In large diesel engines of the crosshead type used in marine and heavy stationary applications, the cylinders are lubricated separately from the other engine components. The cylinders are lubricated on a total loss basis with the cylinder oil being injected separately to quills on each cylinder by means of lubricators positioned around the cylinder liner. Oil is distributed to the lubricators by means of pumps, which are, in modern engine designs, actuated to apply the oil directly onto the rings to reduce wastage of the oil.
One problem associated with these engines is that their manufacturers commonly design them to use a variety of diesel fuels, ranging from good quality high distillate fuel with low sulfur and low asphaltene content to poorer quality intermediate or heavy fuel such as marine residual fuel with generally high sulfur and higher asphaltene content.
The high stresses encountered in these engines and the use of marine residual fuels creates the need for lubricants with a high detergency and neutralizing capability even though the oils are exposed to thermal and other stresses only for short periods of time. Residual fuels commonly used in these diesel engines typically contain significant quantities of sulfur, which, in the combustion process, combine with water to form sulfuric acid, the presence of which leads to corrosive wear. In particular, in two-stroke engines for ships, areas around the cylinder liners and piston rings can be corroded and worn by the acid. Therefore, it is important for diesel engine lubricating oils to have the ability to resist such corrosion and wear.
Accordingly, a primary function of marine diesel cylinder lubricants is to neutralize sulfur-based acidic components of high-sulfur fuel oil combusted in low-speed 2-stroke crosshead diesel engines. This neutralization is accomplished by the inclusion in the marine diesel cylinder lubricant of basic species such as metallic detergents. Unfortunately the basicity of the marine diesel cylinder lubricant can be diminished by oxidation of the marine diesel cylinder lubricant (caused by the thermal and oxidative stress the lubricant undergoes in the engine), thus decreasing the lubricant's neutralization ability. The oxidation can be accelerated if the marine diesel cylinder lubricants contain oxidation catalysts such as wear metals that are generally known to be present in the lubricant during engine operation.
Marine two-stroke diesel cylinder lubricants must meet performance demands in order to comply with the severe operating conditions required for more modern larger bore, two-stroke cross-head diesel marine engines which are run at high outputs and severe loads and higher temperatures of the cylinder liner. Therefore, there is a need for marine diesel cylinder lubricating oil compositions having improved detergency and high heat stability at high temperatures.
Presently, generic design changes in large bore low-speed two-stroke engines as well as changes in operations (both driven by fuel efficiency) have contributed to the frequent occurrence of severe cold corrosion. Cold corrosion is caused by sulfuric acid. The sulfur oxides that result from combustion of the fuel (typically a Heavy Fuel Oil with >2 wt % Sulfur) will, with the water formed during combustion and the water from the scavenge air, form sulfuric acid. When the liner temperature drops below the dew point of sulfuric acid and water, a corrosive mixture is condensed on the liner. Cylinder lubricant basicity, cylinder lubricant feed rate of the oil to the cylinder liner, engine make and type, engine load, inlet air humidity and fuel sulfur content are among the factors that can influence the amount of cold corrosion. High alkaline lubricants are used to neutralize the sulfuric acids and avoid cold corrosion of piston rings and cylinder liner surfaces. High alkalinity lubricants (e.g., up to 100 BN by the ASTM D2896 test method) are currently being marketed to help overcome severe cold corrosion.
Sulfurized, overbased phenates are known compounds which are widely used in marine applications for their detergency properties and thermal stability. However, low molecular weight alkylphenol compounds such as tetrapropenyl phenol (TPP) are often used as raw materials in the manufacture of these sulfurized, overbased phenates. The process to manufacture overbased phenates generally results in the presence of the unreacted alkylphenol in the final reaction product and ultimately in the finished lubricating oil composition. Recent reproductive toxicity studies have shown that in high concentrations of unreacted alkylphenol, TPP in particular, may be endocrine disruptive materials which can cause adverse effects in male and female reproductive organs.
To reduce any potential health risks to customers and avoid potential regulatory issues, there is a further need to reduce or eliminate the amount of unreacted TPP and its unsulfurized metal salt present in lubricating oil compositions. Therefore, it would be even more desirable to develop a marine diesel cylinder lubricating oil composition that is substantially free of unreacted TPP and its unsulfurized metal salt.